Star scores are an attempt to map a qualitative experience (enjoyment of the film) with some quantitative measure. Which is fine if you just want to get a sense of 'how much' somebody liked a something. If I say I give scotch A a 5 scotch B a 3 and scotch C a 4 then you know that I like the scotch's in A, C, B order. It's a short hand way to express my personal ordering of qualitative experience, just like we use the words: 'good', 'better', 'best'.
The problem is this data is not really numerical, so even basic mathematical operations don't make any sense. When we add 2 heights, 2 masses, 2 speeds etc the result makes sense. But not so with ratings. Even basic difference doesn't make sense, is the difference between 5 and 4 stars the same as between 4 and 3 stars? There is no 'unit' distance in scoring system. So doing any sort of averaging is just going to give you nearly meaningless results.